This invention relates to apparatus for chipping wood chips used to make pulp which in turn is used in papermaking machines to make paper and paperboard products. More particularly, this invention relates to apparatus for receiving oversize wood chips and rechipping them into chips having acceptable (i.e. thinner) thickness, but substantially the same length and width.
In the paper industry, wood pulp is made by subjecting wood chips to a chemical process wherein the compounds and chemical systems holding the fibers together, such as lignin, to form the chip are dissolved to thereby liberate the individual wood fibers which are then diluted with water and introduced into a papermaking machine to make the paper or paperboard products. If the wood chips introduced into the refiners in which the chemical fiber liberating process takes place are not of a relatively uniform thickness, within predetermined limits, some chips might not be penetrated by the chemicals at all, or not penetrated for a time sufficient to liberate all the wood fibers. Other chips, if they are too thin, might be exposed to the fiber liberating chemicals for a time longer than necessary to merely liberate the individual fibers whereupon the fibers themselves would be deleteriously weakened, or shortened, or both. Thus, it is very important that the thickness of the chips sent into the pulping digester be uniform within specified limits determined by the kind of wood and desired pulp parameters. Since the chipping equipment operates against the external surface of the logs being chipped, it is relatively easy to control the chip length which coextends substantially with the wood grain along the surface of the generally cylindrical log.
However, the thickness of the individual wood chips is in the direction extending radially inwardly to the center of the log. In other words, the chip thickness might generally be described as extending in a direction normal to an imaginary plane tangent with the generally cylindrical surface of the log periphery. The thickness of the chips produced is therefore more difficult to contol since they sometimes are gouged or broken out in chunks. The chips produced by the chipping apparatus are screened and classified. Oversized chips, sometimes called "cards" in the industry, have heretofore been sent to one of several types of known rechippers. For example, a so-called disk-type rechipper operates by rotating a disk containing a plurality of blades in its face against a stationary bed knife. Gravity fed chips are discharged upwardly under the impetus of the rotating disk blades.
Other types of known rechippers include the rigid-hammer type shredder which utilizes a punch and die type of action wherein teeth mounted on a rotating shaft rotate through slots in stationary anvils. The swing-hammer type shredder utilizes a plurality of pivotally mounted hammers which rotate and force chips through a grid-like breaker plate.
All of these prior types of rechippers/shredders have a common characteristic in that their knives, blades and hammers engage the chips in a random manner which results in the chips being cut, broken and pulverized so that the smaller chips produced have undesirable shorter lengths as well as thinner thickness. A great deal of undesirable fines (i.e. dust and very small chips and pieces) are produced as well.
Operators of these prior types of rechippers attempted to improve the quality of chips produced by the rechippers by feeding them only the very largest chips screened from the chippers, which may be 10% of the output, but this has the concomitant effect of reducing the effective output of the chippers.